This invention relates to the operation of refrigeration systems having self-defrosting apparatus, and in particular to operating such systems so that part of the system collects waste energy during the normal operation thereof to be used at the time defrost is required.
Prior refrigeration systems often have used an outside energy source to effect defrosting when that became necessary due to accumulation of frost or ice on the cooling coils of the system. This is particularly true in plants that have only one freezer, since no other load exists to aid in defrosting the freezer coils. Some systems have used electric resistance heating, which is very expensive and inefficient in the current energy-conscious environment. Other systems have used a separate air unit in the heated space as a load to generate hot gas for use on defrost. While effective, this is not much more efficient or cost effective than electric resistance defrost.
Another approach has been to attempt to split or zone the cooling coils and defrost one part thereof at a time using the energy generated by the rest of the cooling coils, still in the cooling mode. This is difficult to carry out and not very effective because so much air is moving through the freezer that the overall temperature thereof may be raised. To avoid this effect, dampers would have to be used to isolate the portion of the coil being defrosted from the others not being defrosted. Such dampers have been found to be not very dependable. Alternatively, if the refrigeration system is being employed in a line-type application, the defrost must be done during a break period, when the product flow has stopped. This procedure may not be satisfactory since the line may not need to be stopped for any other reason than periodic defrost if it runs continuously. Further, even if the defrost operation is conducted during a break period, use of one coil as a load to defrost another may not be satisfactory since, without product load, the coil not being defrosted may not provide sufficient heat to defrost the other coil.
A defrosting device with a heat extractor or reservoir is disclosed in Redfern, U.S. Pat. No. 3,838,582. That system however is a single stage system, having only one compressor therein. Further, two separate coils are disposed in the energy reservoir. A need exists for a two stage system, and a system that makes more efficient use of materials.
This invention relates to improvements over the apparatus referred to above, as well as to solutions to the problems raised thereby.